1. Field of the Invention
The present invention relates generally to a liquid developing apparatus for developing an electrostatic latent image in facsimile machines, copying machines, printers or the like. In particular, the invention concerns a device or apparatus for controlling concentration in the liquid developing machine so that concentration of a liquid developer be constantly maintained to be constant.
2. Description of the Prior Art
In a facsimile machine and the like, an electrostatic latent image formed on a recording medium such as a sheet of paper is developed by a liquid developer having fine colored particles termed toner dispersed in a solvent to be thereby made visible. Repetition of development results in decreasing the quantity of toner in the solvent, lowering concentration of the developer. Accordingly, in order to maintain concentration of development (i.e. concentration of developer as well as density of developed image) in constant, it is necessary to supplement toner to the solvent in an appropriate manner. FIG. 1 of the accompanying drawing shows a typical one of the hitherto known developing machines equipped with a concentration controlling apparatus for maintaining constant the concentration of a liquid developer.
Referring to FIG. 1, the known liquid developing machine includes a lift pump 2 for feeding a developing liquid (also referred to as liquid developer) 3a contained in a tank 3 to a developing container 1 through a pipe 10. An electrostatic latent image on a recording sheet 8 is developed by the developing liquid fed to the developing container 1.
A pipe 11a is branched from the pipe 10 through which the developing liquid is fed upwardly and is equipped with a concentration detecting device 4 which is composed of a transparent pipe section 4a combined with a light emitting element 4b and a photoelectric sensor element 4c disposed on opposite sides of the transparent pipe section in diametrical opposition to each other. In the developing operation, the developing liquid 3a also flows through the branch pipe 11a, whereby concentration of the developing liquid is detected. The pipe 11a is connected to a toner supplementing apparatus composed of a Venturi tube 5, a valve 7 and a pipe 9. Under the pressure of the liquid developer 3a flowing through the pipe 11a, a negative pressure or vacuum is produced by the Venturi tube 5 so that a negative pressure prevails within the pipe 9. The other end of the pipe 9 is immersed in a pool of concentrated toner 6. Thus, when a valve 7 is closed, the concentrated toner 6 is supplementarily supplied to the tank or container 3 by way of the pipe 9, the Venturi tube 5 and a pipe 11b.
The concentration detecting device 4 and the valve 7 are connected to a concentration controlling circuit 26a. When the developing liquid flowing through the pipe 11a has attained a predetermined level of concentration, the concentration detector responds to activate the concentration controlling circuit 26a in such a manner that the valve 7 is opened. In this state, the ambient air flows through the valve 7, causing the supplementary supply of the concentrated toner 6 to be stopped. The concentration controlling circuit 26a is composed of an amplifier circuit for amplifying the output signal of the concentration detector 4, a comparator for comparing the amplified output signal of the concentration detector 4 with a reference voltage and an electromagnetic valve controlling circuit for controlling the closing and opening operations of an electromagnetic or solenoid valve 7 in dependence on the output signal of the comparator. More specifically, when the amplified output signal of the concentration detector 4 exceeds the predetermined reference voltage level in the comparison by the comparator, a control signal is supplied to the electromagnetic valve controlling circuit which responds thereto for producing a signal which triggers the opening or closing operation of the valve 7.
FIG. 2 is a timing chart for illustrating the timing in the operation described above. In the figure, the output signal of the amplifier circuit is shown at A, the reference voltage value is indicated by V.sub.O, and the output signal of the electromagnetic valve controlling circuit is shown at B. During a period T.sub.O from the detection of a decrease in concentration of the liquid developer to the detection of an increase in concentration, the valve 7 is closed. As is shown in FIG. 2 at A, when the valve 7 is closed during the period T.sub.O, the concentrated toner 6 is added to the developing liquid 3a, as the result of which the amount of light transmitted through the transparent pipe section 4a is decreased. Consequently, the output level of the detecting signal amplifying circuit is correspondingly lowered, indicating that concentration of the developing liquid 3a is increased.
The hitherto known liquid developing machine is however disadvantageous in that the negative pressure or vacuum can not be generated with high effiency or sensitivity due the such structure in which the Venturi tube 5 is connected in parallel with the developing container 1 on the discharge side of the lift pump 2 so that the negative pressure is produced upon flowing of the discharged liquid developer 3a. Further, disposition and connection of the Venturi tube and the pipes 10a, 11a and 11b involve a complicated structure, an increased size of the machine and an increased cost thereof.
Further, the concentration controller employed in the known liquid developing machine described above is so arranged as to actuate or close the valve 7 in response to detection of lowered toner concentration of the developing liquid to thereby cause the concentrated toner to be added to the developing liquid under the action of the Venturi tube, and upon detection of the toner concentration having reached the reference value, the valve 7 is actuated (opened) to stop the addition of the concentrated toner 6. With this arrangement, the concentrated toner flowing though the pipe 9, the Venturi tube 5 and the pipe 11 at the time when the valve is opened is supplied to the developer tank 3 in excess, which may bring about an excessively high density of the toner particles in the liquid developer 3a. As a consequence, background fog and condensation of toner particles often take place, which is a serious problem.
Further, when the known developing machine is left unused for a long time after a developing operation, solvent in the developing liquid sticking to the wall of the transparent pipe 4a of the concentration detector 4 will be evaporated, resulting in the toner being solidified on the wall so as to decrease the transmittivity of the transparent pipe section 4a below a nominal value. In that case, the amount of light impinging on the photoelectric sensor element 4c cannot be increased regardless of a decrease in concentration of the developing liquid, whereby addition of toner to the developing liquid does not take place. Then, concentration of the developing liquid is lowered below the nominal level, which is another disadvantage.